While Uncle Tom's Cabin by Harriet Beecher Stowe is often viewed as a factor that pushed the United States toward a Civil War over the issue of slavery, the impact of the novel has carried on more than 150 years after the Civil War. Today, the phrase "Uncle Tom" is still used as a derogatory word for African Americans. It's often used to describe someone who betrays the African American community or someone who is too submissive to white people in a position of power.
The phrase is sometimes given to famous musicians, athletes, and other African Americans with a platform to speak on social issues who don't use that platform to shine a light on the systemic racism that still exists. Even Civil Rights leaders like Martin Luther King Jr. and Rosa Parks who went about things in a nonviolent fashion were given label of "Uncle Tom" because of their passive form of resistance of Jim Crow laws.
However, one could argue that the phrase "Uncle Tom" has been taken out of context from Stowe's novel. In the book, Stowe writes that Tom has a "grave and steady good sense" while giving him the physical description of "a large, broad-chested, powerfully-made man." Moreover, he doesn't betray his fellow slaves in the novel. At one point, he endures a whipping from his master rather than give away the location of two slaves who fled.
The issue is that the play Uncle Tom's Cabin, which remained popular for decades following the Civil War, depicted the character as more submissive. That version of Tom became more sentimental than strong, leading to negative connotations being associated with the phrase "Uncle Tom."
Despite how the phrase "Uncle Tom" has changed connotations over the last 150 years, it remains part of the lasting legacy of Stowe's novel. One can't use the phrase "Uncle Tom" without knowing that it's a reference to one of the most important depictions of slavery ever written.